The present invention relates to an improved optical fiber communications cable. More particularly, the invention relates to a fiber optic ribbon cable in which water is prevented from longitudinally flowing along the interior of the fiber optic ribbon cable.
In order to overcome axial water penetration through a buffer tube in a fiber optic cable, it has been the practice in prior cable designs to flood the buffer tubes with a grease or gel in order to fill the interstices of the buffer tube(s) of a fiber optic cable so as to prevent the flow of water along the axis of the cable. Most conventional fiber optic cables for outdoor use that include fiber optic ribbon cables and loose tubes utilize the aforementioned grease or gel flooded buffer tube technology.
As is also well known to those skilled in the fiber optic cable art, the flooding of the buffer tube(s) of a fiber optic cable with grease or gel to prevent axial water penetration through a buffer tube suffers from many shortcomings and drawbacks. The most serious shortcoming is that a craftsman handling the optical fiber communications cable must remove the grease or gel from the buffer tube(s) and clean all of the elements therein before beginning whatever work is necessary for the craftsman to perform. The cleaning process is both time consuming and unpleasant for the craftsman. Also, the grease or gel adds a significant amount of weight to the fiber optic communications cable which results in limiting its potential uses. For these and other reasons, it is desirable to eliminate the need to flood the buffer tube(s) of a fiber optic cable with grease and gel to prevent axial water penetration through the buffer tube(s) of the fiber optic cable.
Also, efforts have been made to deal with the water penetration and water migration problem of fiber optic cable by using superabsorbent compounds which are added to an optical fiber cable in the form of treated yarns, tapes and/or core elements as well as applying thin coatings and powders selectively to the cable construction. U.S. Pat. Nos. 4,703,998; 4,913,517; 5,133,034; and 5,642,452 (assigned to the assignee of the present invention) are representative patents relating to the use of superabsorbent compounds in optical fiber cable constructions to impart water blocking capabilities to the optical fiber cables. However, the prior efforts to utilize superabsorbent compounds have not all proven to be highly effective at a modest cost for reasons that are well known to those skilled in the fiber optic cable art.
The present invention is believed to overcome the foregoing problems of the prior art by providing for a novel fiber optic ribbon cable construction as described herein.
In accordance with the present invention, applicants provide a water-blocked optical fiber loose tube cable possessing enhanced performance characteristics. The cable comprises a plurality of stacked fiber optic ribbons having a plurality of water blocking yarns extending generally along the length of the stack of fiber optic ribbons and positioned around at least a portion of the circumference of the stack wherein the plurality of water blocking yarns possess water swellable characteristics. The stack of fiber optic ribbons and the plurality of water blocking yarns extending along the length of the stack of fiber optic ribbons are all loosely disposed in a buffer tube having an interior channel larger than the stack of fiber optic ribbons. The swell capacity (Mcapacity) of the plurality of water blocking yarns should exceed the critical mass of water (Mcritical) that could enter the buffer tube by a factor of 2.0 or more wherein the swell capacity is determined by the formula:
Mcapacity=(N)(d)(B)
wherein
N=number of water blocking yarns
d=yarn denier
B=absorbency
and the critical mass of water is determined by the formula:
Mcritical=(Atubeopen)(Pwater)
wherein
Atubeopen=open area of buffer tube
Pwater=density of water
Optionally, a suberabsorbent powder may also be applied between and/or on the stack of fiber optic ribbons loosely disposed in the buffer tube of the optical fiber loose tube cable.
It is therefore an object of the present invention to provide an improved water-blocked optical fiber communications cable.
It is another object of the present invention to provide an improved water-blocked fiber optic ribbon cable wherein cable weight is significantly reduced vis-à-vis grease or gel cable construction so as to enhance installation of the cable by a craftsman and allow installation of longer cable lengths.
It is another object of the present invention to provide an improved water-blocked fiber optic ribbon cable that can be used for outside plant data communications and telecommunications applications without using grease or gel filling compounds within the buffer tube(s).
Some of the objects of the invention having been stated hereinabove, other objects will become evident as the description proceeds, when taken in connection with the accompanying drawing as best described hereinbelow.